1. Field of the Invention
The present invention generally relates to a method of testing integrated circuits. More particular, the present invention relates to a method of monitoring ion contamination in MOS integrated circuits. By providing steady voltage biases and high temperature, the mobile ions are driven into or out of the gate oxide layers of MOS transistors in integrated circuits. The threshold voltages of MOS transistors are measured. Therefore, the situation that an integrated circuit is contaminated by ions can be reflected.
2. Description of the Related Art
MOS devices have been widely applied for fabricating and designing integrated circuits, because of the features of their low power dissipation and high integrity. During the process of fabricating semiconductor integrated circuits, the integrated circuits are inevitably be contaminated by ions, such that the performance and characteristic of MOS transistors are degraded. For a MOS transistor in an integrated circuit, if its gate oxide is contaminated by ions, then the threshold voltage will deviate from the original designed value, resulting in error operation. Taking static random access memory (SRAM) for example, a SRMA cell comprises at least four NMOS transistors formed in a P type substrate. If any NMOS transistor in the cell is contaminated by ions, then, when the cell works in steady bias voltage, the ions will be driven toward the region beneath the gate oxide layer, resulting in flat band voltage drifting. Therefore, the threshold voltage of an NMOS transistor is reduced, and the data stored in the SRAM cell are lost easily.
At present, two analysis methods, B-T-S-CV and SIMS, are used for examining the above issues. However, the above methods can not reflect the characteristics of MOS devices directly. Furthermore, the C-V (capacitance-voltage) characteristic is tested through test keys of MOS devices, after completing integrated circuits. However, the ion contamination induced in the processes after forming gates of MOS transistors can not be reflected accurately.